This invention relates to a process for imaging a lithographic printing plate and more particularly to a process using an ink jet printer to imagewise apply an insolubilizing chemical to a plate coating which comprises photosensitive compounds or their reaction products.
In the art of lithographic printing it is generally required that one or more lithographic printing plates be mounted on a printing press. The lithographic printing plate is characterized by having on its printing surface oleophilic ink receiving areas in the form of the image to be printed, and hydrophilic water receiving areas corresponding to the other, non-printing areas of the surface. Because of the immiscibility of oil-based lithographic inks and water, on a well-prepared printing plate, ink will fully coat the oleophilic areas of the plate printing surface and not contaminate the hydrophilic areas. The operating press brings the inked plate surface into intimate contact with an impression cylinder or elastic transfer blanket that transfers the ink image to the media to be printed.
Traditionally, a lithographic plate is photographically imaged. The plate substrate is most commonly aluminum, from 5 to 12 mils thick, treated so that the printing surface is hydrophilic, although treated or untreated plastic or paper substrates can also be used. The substrate is coated with a solution of a photosensitive composition that is generally oleophilic. Upon drying, the coating layer thickness is commonly about 1 to 3 microns thick. A printing plate with such a photosensitive coating is called xe2x80x9cpresensitizedxe2x80x9d (PS). Both negative and positive working photosensitive compositions are used in PS lithographic plates. In a negative plate, light exposure insolubilizes the coating, so that on development the only parts of the coating that aren""t removed are the light imaged areas. The reverse is the case in a positive plate. Light exposure solubilizes the coating; on development the coating is only removed in the areas that are light imaged. In an image reversal process, a positive plate is xe2x80x9cblanket exposedxe2x80x9d or xe2x80x9cflood exposedxe2x80x9d, i.e., the entire plate is light exposed without any intervening mask or other means for imaging, and imaged in a separate step which can be performed before or after the blanket exposure step. By this image reversal process, a positive plate can be negatively imaged. The aluminum substrate can be treated to make it hydrophilic either prior to the application of the photosensitive composition or at the time the non-image areas of the coating are removed in a development step. Such a process in which a pre-coated lithographic plate is prepared for press by removing exclusively either the imaged or non-imaged coating in a development step is called a subtractive process; a pre-coated plate having a coating which is at least partially removed in a development step is known as a subtractive plate.
Photosensitive compositions used in positive lithographic plates are well known. They. are comprised primarily of alkali soluble resins and o-quione diazide sulfonic acid esters or amides. In addition dyes or colored pigments, indicator dyes, plasticizers and surfactants can also be present. The ingredients are typically dissolved in organic solvents and are coated onto the substrate. Upon drying a thin film or coating is produced.
Alkali soluble resins useful in positive plates are well known and include phenol-formaldehyde resins, cresol-formaldehyde resins, styrene-maleic anhydride copolymers, alkyl vinyl ether-maleic anhydride copolymers, co-or ter-polymers that contain either acrylic or methacrylic acids and poly(vinyl phenol). U.S. Pat. No. 4,642,282 describes an alkali soluble polycondensation product that is also useful as the resin component in positive plates.
The o-quinone diazide compounds include o-benzoquinone diazides, o-naphthoquinone diazides and o-anthraquinone diazides. O-quinone diazide compounds useful in positive plates are well known and are described in detail in Light Sensitive Systems by J. Kosar, p.339-352. They are further described in U.S. Pat. Nos. 3,046,118; 3,046,119; 3,046,120; 3,046,121; 3,046,122; 3,046,123; 3,148,983; 3,181,461; 3,211,553; 3,635,709; 3,711,285 and 4,639,406 incorporated in entirety herein by reference.
Such positive plates are sensitive to light in the wavelength range of from about 290 to 500 nm. When used in the standard manner, photo-exposure causes the alkali insoluble o-quinone diazide of the positive plate to be converted into an alkali soluble carboxylic acid. Upon subsequent treatment with a developer, which is a dilute aqueous alkaline solution, the exposed parts of the coating are removed. The unexposed coating is alkali insoluble, because the o-quinone diazide is unaffected by the developer, and remains on the substrate.
Traditionally, lithographic plates are imaged by photographic transfer from original artwork. This process is labor-intensive and costly. Hence with the advent of the computer engendering a revolution in the graphics design process preparatory to printing, there have been extensive efforts to pattern printing plates, in particular lithographic printing plates, directly using a computer-controlled apparatus called a platesetter that is supplied with digital data corresponding to the image to be printed. A platesetter has the capability to supply an image forming agent, typically light energy or one or more chemicals, to a plate according to various patterns or images as defined by digital data, i.e., to imagewise apply an image forming agent. Specially manufactured lithographic plates may be required for certain types of platesetters. Such a combination of a computer-controlled platesetter and the proprietary plates used with them along with developer solutions and any other materials or apparatuses necessary to prepare the plates for printing is known as a computer-to-plate (CTP) system.
Heretofore, many of the new CTP systems have been large, complex, and expensive. They are designed for use by large printing companies as a means to streamline the prepress process of their printing operations and to take advantage of the rapid exchange and response to the digital information of graphic designs provided by their customers. Many of the new CTP systems use light sources, typically lasers, to directly image PS plates. But using lasers to image plates is very expensive, because the per-unit cost of the lasers is high and because they require sophisticated focusing optics and electronic controls. If because of the cost only a single laser is used, then time becomes a constraint because of the necessity of raster scanning. There remains a strong need for an economical and efficient CTP system for the many smaller printers who utilize lithographic printing.
In recent years, ink jet printers have replaced laser printers as the most popular hard copy output printers for computers. Ink jet printers have several competitive advantages over laser printers. One advantage is that it is possible to manufacture an array of 10""s or even 100""s of ink jet nozzles spaced very closely together in a single inexpensive print head. This nozzle array manufacturing capability enables fast printing ink jet devices to be manufactured at a much lower cost than laser printers requiring arrays of lasers. And the precision with which such a nozzle array can be manufactured and the jetting reliability of the incorporated nozzles means that these arrays can be used to print high quality images comparable to photo or laser imaging techniques. Ink jet printers also are increasingly being used for prepress proofing and other graphic arts applications requiring very high quality hard copy output.
In spite of the large and rapidly growing installed base of ink jet printers for hard copy output, ink jet printing technology is not commonly used in CTP systems. There are many challenging technical requirements facing the practitioner who would design such an ink jet based CTP system as can be seen in the prior art. A first requirement is that the ink jet ink used to image the printing plate be jettable, able to form ink drops of repeatable volume and in an unvarying direction. Further, for practical commercial application, the ink must have a long shelf life, in excess of one year or more. U.S. Pat. No. 5,970,873 (DeBoer et al) describes the jetting of a mixture of a sol precursor in a liquid to a suitably prepared printing substrate. But any ink constituents of limited solubility will render unlikely the practical formulation of a jettable, shelf-stable ink. Similar problems exist in U.S. Pat. No. 5,820,932 (Hallman et al) in which complex organic resins are jetted, and U.S. Pat. No. 5,738,013 (Kellet) in which marginally stable transition metal complexes are jetted. In U.S. Pat. No. 6,187,380 B1 (Hallman et al) and U.S. Pat. No. 6,131,514 (Simons), inks comprising acrylic resins such as trimethylolpropanetriacrylate and poly(ethylene-co-acrylic acid, sodium salt), are jetted. While it may be possible to make such a ink formulation work for the purposes of a short term experiment, it would almost certainly clog the nozzles of an ink jet printhead were the ink allowed to remain in the printer for the weeks or more that would be a requirement of practical commercial use.
Another requirement is that to be of wide utility, the ink jet based CTP system should be able to prepare printing plates with small printing dots, approximately 75 microns in diameter or smaller, so that high resolution images can be printed. Ink jet printers can produce such small dots, but of those having substantial commercial acceptance, only ink jet printers employing aqueous-based inks are practically capable of printing such small dots. Thus the systems described in U.S. Pat. No. 4,003,312 (Gunther), U.S. Pat. No. 5,495,803 (Gerber), U.S. Pat. No. 6,104,931 (Fromson et al), and U.S. Pat. No. 6,019,045 (Kato) which use solvent-based hot melt inks will not allow the preparation of the high resolution printing plates necessary for printed images of high quality. Further, hot melt type inks typically freeze on top of the imaged media rather than penetrate into it. This would prevent intimate mixing between potential reactants in the inks and corresponding potential reactants in a PS plate coating. It is also required that the prepared printing plates be rugged, capable of sustaining press runs of many thousands of impressions. The waxes used in the hot melt inks described in U.S. Pat. No. 6,019,045 (Kato) and U.S. Pat. No. 4,833,486 (Zerillo) would wear out in such a long press run.
Another requirement of a successful inkjet based CTP system is that a mature plate technology is to be preferred. Although the prior art demonstrates that it is not obvious to do so, it greatly simplifies the development of an ink jet CTP system to be able to use commercially available, widely accepted PS plates. There are many tradeoffs in the manufacture of commercially practical lithographic plates. They must be highly sensitive to the imaging process and yet thermally stable, stable in high humidity storage environments and yellow light, resistant to fingerprints, of minimal toxicity and environmentally benign, easily developed in that small dots are quantitatively resolved without dot blooming using developers that are of minimal toxicity and environmentally benign, able to sustain long press runs, manufacturable at a low cost per square foot, and many other practical requirements. U.S. Pat. No. 5,695,908 (Furukawa) describes a process for preparing a printing plate comprising a new plate coating containing a water-soluble polymer that becomes water-insoluble in contact with a metal ion in a solution jetted imagewise. But such a new plate coating is unlikely to meet the wide array of constraints on a successful plate technology. U.S. Pat. No. 5,466,653 (Ma et al) describes a plate coating that requires an impractically high reaction temperature for imaging. U.S. Pat. No. 6,025,022 (Matzinger) describes a new plate coating on a glass substrate that would be unlikely to find wide acceptance.
To use an ink jet printer in a positive imaging process is impractical because in typical printing, the area of a plate containing images such as text, graphics, and line work, is much less that the non-image containing area of the plate. Thus to be able to image widely accepted positive plates with a negative imaging ink jet process is a unique, surprising, and valuable result.
Positive plates based on o-naphthoquinone diazide sulfonic acid esters can be modified by the incorporation of alkaline materials to obtain image reversal. U.S. Pat. No. 4,104,070 describes the use of imidazolines; U.S. Pat. No. 4,196,003 describes the addition of secondary and tertiary amines and U.S. Pat. No. 4,356,254 describes the addition of basic carbonium dyes to produce image reversal. The sequential steps for this image reversal process are imagewise light exposure, heat treatment, blanket light exposure and alkaline development. Those coatings have never achieved any commercial success, which is attributed to the adverse effect on the properties of the coating by the addition of the alkaline materials. U.S. Pat. No. 4,007,047 describes image reversal of a positive resist by a modification of the photoimaging process. After imagewise exposure, the resist coating is subjected to an acid treatment by immersion into a heated acid solution, which after a water rinse and drying steps produces a negative image after blanket light exposure and development.
The present invention provides a process for preparing PS lithographic plates for printing by employing an ink jet printhead to imagewise apply an insolubilizing chemical. In one aspect of the invention, the steps comprise:
(a) providing an on-press developable PS plate;
(b) imagewise applying an insolubilizing chemical to the plate coating;
(c) heating the plate;
(d) mounting the plate on a printing press; and
(e) operating the press.
In another aspect of the invention, the steps comprise:
(a) providing a positive plate;
(b) blanket exposing the plate;
(c) imagewise applying an insolubilizing chemical to the plate coating;
(d) heating the plate; and
(e) developing the plate.
In yet another aspect of the invention, the steps comprise:
(a) providing a plate with a coating;
(b) imagewise applying an insolubilizing chemical to the coating;
(c) heating the plate; and
(d) washing the plate with a developing solution.
Accordingly, there are several objects and advantages of the present invention. An object of the present invention is that it is easily embodied in a practical, reliable, and inexpensive ink-jet-based CTP system with minimal constraints on the formulation of the imaging fluid, and in that widely-available, commercially-accepted lithographic plates with desirable aluminum substrates and their corresponding commercially-accepted development solutions can be used without modification. A plate prepared by the present invention is functionally similar to a plate prepared by photographic imaging, with a potential long run life but without the complexity, cost, or waste of a film negative. The plate can be prepared quickly, in that fewer steps are required and in that a speedy ink jet printer can be employed, and yet the plate image is of high resolution, enabling high quality 4-color printing. Further, the plate thus prepared is storage stable, little subject to contamination in its hydrophilic areas and can be used on a wide variety of commercially available and accepted printing presses. The formulation of the insolubilizing fluid is flexible and can be simple, inexpensive to manufacture, environmentally safe, and non-toxic. Water can be used as a vehicle resulting in small drops that easily penetrate PS coatings facilitating intimate mixing of the insolubilizing chemical with the reactive constituents of the coating. The chemicals used in the fluid in this invention that enable insolubilization to occur on negative and photo-exposed positive plates cover a wide range of materials. That such a simply and flexibly formulated ink can be jetted in very small diameter drops to produce high resolution images on conventional and widely accepted positive and negative working plates is a unique and surprising result. That commercially available positive plates can be economically and efficiently imaged by ink jet in a negative working process is a unique and surprising result. Printers already using positive plates for printing requiring light imaging exposure can now use efficient and economical ink jet negative imaging without having to switch plate types.
Still other objects and advantages will become apparent from the claims, and from a consideration of the ensuing detailed description of the invention.